The present invention relates to a bistable straight line motion actuator of a type suitable for actuating a poppet valve in an internal combustion engine. More particularly, the invention relates to an electronically controlled, hydraulically powered actuator which utilizes a pneumatic spring for energy recovery, and hydraulic latching.
A pneumatically powered actuator with hydraulic latching is disclosed in U.S. Pat. No. 5,022,359, which patent is incorporated herein by reference. This patent gives a thorough discussion of prior art actuators, particularly pneumatically powered actuators with energy recovery using compressed air. Virtually all of the prior art actuators discussed in the patent use some type of magnetic latching for holding the actuator in one of two stable positions.
U.S. Pat. No. 5,022,359 discloses a mechanism which uses a low air pressure (about 10 psi) to hold a working piston in its first stable position (engine valve closed). When a magnetic control valve is electronically switched, high air pressure (about 100 psi) drives the piston toward its second stable position compressing the air in front of it. This motion admits hydraulic fluid to an expansion chamber via a ball check. When the piston reaches its second stable position, the control valve has returned to its initial state, cutting off the air supply, and the compressed air behind the piston is released to atmosphere. The air in front of the piston is fully compressed, but the ball check closes and hydraulic fluid in the expansion chamber prevents motion back toward the first stable position, thereby maintaining the engine valve open. At the conclusion of the valve dwell, an electronically controlled magnetic plunger forces the ball check open, and the compressed air (stored potential energy) forces the piston back toward its first stable position. Air is compressed in front of the moving piston to dampen its motion, but this air is released just as the piston reaches its first stable position.
The actuator mechanism disclosed in U.S. Pat. No. 5,022,359 represents an improvement over the prior art insofar as propulsion air is used only to open the engine valve, and not to close it. The compressed air consumed is therefore decreased to about half the air consumed in prior pneumatically powered systems. However, two separately controlled magnetic mechanisms, one for the air control valve and one for the plunger to release the ball check, are required. Since the air control valve is rather large, a large electromagnetic latch is required. Further, due to the time required to pressurize the piston with air, after the control valve is switched, the response time is slow and not suited to use at high RPM.
U.S. application Ser. No. 07/878,644 filed May 5, 1992 discloses a fully symmetric pneumatically powered actuator wherein a working piston is pneumatically driven by opposed sources of compressed air in two opposed directions, and hydraulically latched in opposed stable positions by a two position hydraulic latch which is the sole electronically controlled component.
The latch is in effect a two-directional check valve which in each position admits fluid to a respective hydraulic chamber to prevent reverse movement of the working piston. When the check valve is electronically switched, hydraulic fluid passes between the two hydraulic chambers and the latch is released, permitting one of the sources of compressed air to drive the working piston as a working chamber behind the piston expands. As the piston moves, the source of compressed air connected to the expanding working chamber is cut off. Shortly after this, the compressed air expanding in the working chamber is exhausted through ports exposed by the piston. Meanwhile, air is compressed in a working chamber in front of the piston, which working chamber is connected to another source of compressed air in the final stage of movement. This provides damping for the piston without any additional loss of air or air pressure.
The two sources of compressed air are actually just cavities connected to a single source of air which replenishes air lost from an expanding working chamber through the exhaust ports after work is done. The small amount of make-up air is provided when each cavity is connected to its working chamber by action of the advancing piston.
The actuator according to this application is simpler than that disclosed in U.S. Pat. No. 5,022,359 insofar as only one electronically actuated magnetic latch is needed. Since this latch is only moving a low mass valve of the two-way check valve, the magnets are relatively small as compared to most prior art arrangements. Due to the low mass of the check valve, response times are relatively fast.
The two-way check valve provides for hydraulic latching in both stable positions, and at the same time permits a fast response. That is, in addition to the low mass, the high hydraulic pressures created during latching provide for a rapid commencement of movement when the check valve is reversed on electronic command.
While the need for compressed air has been reduced, a continuous supply of air is still required. Further, a rather complex cinching arrangement is required to assure that the engine valve is fully seated.